One of the current trends of development in the storage industry relates to methods and strategies for reduced energy consumption. Data centers can comprise nowadays dozens of storage systems, each comprising hundreds of disk drives. Clearly, most of the data stored in these systems is not in use for long periods of time, and hence most of the disk drives are likely to contain data that is not accessed for long periods of time. Power is unnecessarily spent in keeping all these disk drives spinning and, moreover, in cooling the data centers. Thus, efforts are now being invested in reducing energy-related spending for storage systems. Moreover, environmental regulations are increasingly being enforced in many countries, forcing data centers to adopt “green” technologies for its servers and storage systems.
One obvious way to address the increasing need for reduced energy consumption in storage systems is to manufacture servers and disk drives with reduced levels of energy consumption in normal usage. Vendors are indeed working in this direction, but this issue stands on its own, independent of the question of how the disk drives are used in the storage system, and the question of whether these systems can be designed in a way that reduces energy consumption regardless of the drives being used. Some basic strategies have been developed for regulating energy use within the system, based on the possibility of transitioning the drives to a low-power state when they are not in use, and restoring the normal, or “active” state whenever needed. The disk drive referred to hereinafter as “in low-power state” or “in low power mode” can be adapted to have reduced number of revolutions per minutes (RPM) or can be turned off. Turning the disk drive off can comprise either making it idle (in which case not only does the disk drive not rotate, but also the electronic does not respond), also called “sleep” state, or stopping the rotation but having the electronic respond (also called “stand by” in the industry). Each of these low-power state options has advantages and disadvantages well-known in the art in terms of energy saving, time to return to active state, and wear-off produced by the change in state.
The problems of reduced energy consumption in mass data storage systems have been recognized in the art and various techniques have been proposed as solutions including inter-alia:
US Patent Application No. 2006/0107099 (Pinheiro et al) discloses a redundant storage system comprising: a plurality of storage disks divided into a first subset, wherein all of the plurality of storage disks are dynamically assigned between the first and second subset based on redundancy requirements and system load; a module which diverts read requests to the first subset of storage disks in the redundant storage system, so that the second subset of storage disks in the redundant storage system can transition to a lower power mode until a second subset of storage disks is needed to satisfy a write request; a detection module which detects if the system load in the redundant storage system is high and detects if the system load in the redundant storage system is low; and a module which, if the system load is high, adds one or more storage disks from the second subset to the first subset of storage disks in the redundant storage system so as to handle the system load and if the system load is low, adds one or more storage disks from the first subset to the second subset.
US Patent application No. 2009/129193 (Joshi et al.) discloses an energy efficient storage device using per-element selectable power supply voltages. The storage device is partitioned into multiple elements, which may be sub-arrays, rows, columns or individual storage cells. Each element has a corresponding virtual power supply rail that is provided with a selectable power supply voltage. The power supply voltage provided to the virtual power supply rail for an element is set to the minimum power supply voltage unless a higher power supply voltage is required for the element to meet performance requirements. A control cell may be provided within each element that provides a control signal that selects the power supply voltage supplied to the corresponding virtual power supply rail. The state of the cell may be set via a fuse or mask, or values may be loaded into the control cells at initialization of the storage device.
US Patent application No. 2009/249001 (Narayananet al.) discloses storage systems which use write off-loading. When a request to store some data in a particular storage location is received, if the particular storage location is unavailable, the data is stored in an alternative location. In an embodiment, the particular storage location may be unavailable because it is powered down or because it is overloaded. The data stored in the alternative location may be subsequently recovered and written to the particular storage location once it becomes available.
US Patent application No. 2010/027147 (Subramaniar et al.) discloses a low power consumption storage array. Read and write cycles are separated so that a multiple disk array can be spun down during periods when there are no write requests. Cooling fans are operated with a pulse-width modulated signal in response to cooling demand to further reduce energy consumption.